For three decades now, ionic liquids (ILs), organic salts comprising only ions, have emerged as a new class of pharmaceuticals. Although recognition of the antimicrobial effects of ILs is growing rapidly, there is almost nothing known about their possible virucidal activities. This probably reflects the paucity of understanding virus inactivation. In this study, we performed a systematic analysis to determine the effect of specific structural motifs of ILs on three different biological test systems (viruses, bacteria and enzymes). Overall, the effects of 27 different ILs on two non-enveloped and one enveloped virus (P100, MS2 and Phi6), two Gram negative and one Gram positive bacteria (E. coli, P. syringae and L. monocytogenes) and one enzyme (Taq DNA polymerase) were investigated. Results show that while some ILs were virucidal, no clear structure activity relationships (SARs) could be identified for the non-enveloped viruses P100 and MS2. However, for the first time, a correlation has been demonstrated between the effects of ILs on enveloped viruses, bacteria and enzyme inhibition. These identified SARs serve as a sound starting point for further studies.

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Fatty acid-based ionic liquids (FAILs) are an emerging class of biodegradable ILs. Although long-chained FAILs in particular offer a promising alternative to problematic hydrophobic ILs in lubrication and extraction applications, recent studies have revealed the possibility of an anionic "side-chain length effect", which would adversely impact their ecofriendliness. This study therefore investigated the toxicity of FAILs with sidechain lengths between C 1 and C 18 and benign cations by applying a tripartite biological test system, including enzyme inhibition test, determination of virucidal activity against three model viruses, and bactericidal activity against 12 clinically relevant bacteria. Results do not confirm a general anionic "sidechain effect" with FAILs but rather demonstrate a diverse outcome depending on the biological test system employed. While Gram-negative bacteria and viruses were not affected by increasing FAIL anion alkyl chain length, medium-and long-chained FAILs exhibited pronounced effects at the enzyme level and on Gram-positive bacteria. While this study reinforces the necessity to establish a multilevel approach for the (eco)toxicological evaluation of FAILs, it also demonstrates a specific effectiveness of FAILs against enzymes and Gram-positive bacteria, which should be considered for possible future applications.

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The effective elimination of foodborne pathogens through cleaning and disinfection measures is of great importance to the food processing industry. As food producers rely heavily on disinfectants to control pathogenic bacteria in their facilities, the increasing spread of tolerant, often even multidrug resistant, strains is of particular concern. In addition to efforts to prevent or at least reduce development and spread of strains resistant to disinfectants and sanitizers, there is an urgent need for new and effective antimicrobials. One new class of promising antimicrobials is ionic liquids (ILs), which have been reported to be effective against resistant strains as they interact with bacterial cells in multiple ways, but investigations of their effectivity against MDR bacteria or specific defense mechanisms are still limited. This study investigates the role of multidrug efflux pumps of the Resistance Nodulation-Division family (RND) on the resistance of bacterial pathogens Escherichia coli and Salmonella enterica serovar Typhimurium toward 10 antimicrobial active ILs. Results reveal that, while known structure–activity relationships (SARs), such as the side-chain effect, were found for all strains, antimicrobial ILs with one elongated alkyl side chain were significantly affected by the RND efflux pump, highlighting the importance of efflux pumps for future IL toxicity studies. In case of antimicrobial ILs with multiple side chains and different cationic head groups, two ILs were identified that were highly active against all investigated strains with little to no effect of the efflux pump. The results obtained in this study for RND efflux pumps can serve as a starting point for identifying and designing antimicrobial ILs as effective biocides against MDR bacteria.